Referring to FIG. 1, the use of an outer connection of a standard coaxial cable 50 with a sufficiently high voltage rating and an outer part of a connector 60 such as a PL259 or Type N for high voltage connections is known. The outer layer 51 of the coaxial cable is stripped well back from the inner insulator 52 for suitable voltage creep clearance between the inner and outer conductors and the outer part of the connector 60 is electrically and mechanically connected to the coaxial cable braid. The end of the coaxial inner conductor has a suitable connector 61 fitted for connection to a load device. FIG. 1 shows the use of such a known 4 mm pin coaxial connector.
Magnetrons generate electrical noise from their high voltage terminals. For a typical magnetron this broadband noise can cover the frequency range of ˜100 MHz to ˜600 MHz. Rectifier commutation noise associated with rectifiers used to power magnetrons is usually in the frequency range of ˜1 MHz to ˜20 MHz.
It is well known that inductors can be made by the simple expedient of slipping magnetic cores over a conductor to obtain a desired inductance over a desired frequency range. However, it is unlikely that a single inductor would be effective over the two frequency ranges of the two noise sources outlined above. To filter noise with these different frequency ranges and provide inductors optimised for attenuation in these two different frequency ranges two different inductor core are used in series, which may have respective inductances La and Lb. For each magnetic core the inductance and Q is optimised by a suitable choice of materials for the cores to cover an appropriate frequency range.
GB 1 487 583 and U.S. Pat. No. 3,922,612 disclose a choking circuit comprising a low pass filter for a magnetron which emits VHF and UHF interference in which a pair of choke members is connected between respective heater leads and power input terminals, each choke member being comprised of at least one choke element including a first ferrite core of high intrinsic resistivity, e.g. 10 kΩ·cm, and at least one second choke element of low intrinsic resistivity, e.g. 30 Ω·cm, the two choke members being connected in series. The coke members comprise series connected coils and cores inserted therein. The chokes are located in a shielding enclosure. Provision of the choke members substantially prevents leakage of objectionable microwave noise. A metal shield plate may be inserted between the low and high resistivity cores to obtain magnetic isolation between the cores for more effective absorption of microwave noise. In an embodiment three chokes may be used in series for each lead. Alternatively, the choke members may include two ferrite beads in series around the leads, a first ferrite bead of high intrinsic resistivity and a second ferrite bead of low intrinsic resistivity, each having two bores for receiving the two heater wires. The choke members may comprise a plurality of groups of alternate low and high resistivity ferrite cores.
GB 1 436 928 discloses a magnetron having a cathode lead with a microwave attenuating ferrite bead.
U.S. Pat. No. 4,163,175 suggests that a magnetron in which an electromagnetic energy absorber such as a ferrite bead is wrapped around a cathode holder is well known but the ferrite bead may overheat through the absorption of electromagnetic energy and from the hot cathode causing the ferrite bead to generate a large quantity of gas. There is disclosed a ring-shaped HF energy absorber in contact with a heat conductor in the vicinity of the cathode holder to prevent such overheating of the ferrite inductor.
However, magnetrons can also arc, i.e. generate a short circuit very rapidly. In this situation a full working voltage of a magnetron, which is typically 20 kV, would be transiently applied across the inductors. Such a high voltage could damage the magnetic materials used for the magnetic cores.